1. Field of the Invention
The invention relates to an improved method for producing coatings or shells on a mass of centers, and more particularly, a rapid and more efficient method of panning and film coating to produce coatings or shells on an intermixed mass of centers.
2. Related Background Art
Panning and film coating are related industrial processes used for the preparation of coated compositions. Panning is typically considered to relate to the preparation of sugar-based coatings, whereas film coating is considered to relate to the preparation of non-sugar based (e.g., polymeric) coatings. These coating processes, however, are conducted in the much the same manner. Both panning and film coating are repetitive processes consisting of drying fine layers of a coating solution on an intermixed mass of centers. On a microscopic level, panning is the drying and crystallization of the sucrose or other sugars that may be contained in the coating solution onto the surface of the center or onto the surface of the coated center. Film coating is the drying of cellulose polymers or other conventional film-forming materials contained in the coating solution onto the surface of the center or onto the surface of the coated center. In operation, the panning process comprises a repetition of three cycles: a coating solution application cycle, a distribution cycle, and a drying cycle; the film coating process comprises the simultaneous application and drying of the coating solution. In each process, thin layers of the coating material build upon each other to form the resulting shell or coating.
The centers referred to herein may be any of a variety of pelletized, tabletted, molded or granulated products. Coatings are applied to such centers to seal the center or to add an additional material to the center. Examples of such coated center compositions include pharmaceuticals, such as medicinally coated pills, tablets and non-pareils; chemical products such as detergents; and foodstuffs such as sugar or chocolate coated candies and mints. For pharmaceutical compositions, such coated compositions prevent waste of valuable drugs or medicines contained in the center, and ensures accurate measurement and delivery of dosage. In addition, such coatings serve to protect the centers from degradation or decomposition by exposure to air (oxygen) and/or humidity.
The most common technique used to coat centers is to provide a coating vessel in which a mass of centers are intermixed while dispensing a coating material onto the centers and drying the centers with a current or flow of a drying gas (dry air). Typically, the coating materials are powders such as sugar, sugar alcohols, waxes, and celluloses, or are liquids, most often an aqueous or solvent solution (a coating solution) of sugar, sugar alcohols, waxes, and celluloses. Coating solutions may be prepared by simply dissolving any suitable coating ingredient, or combination of ingredients (e.g., sucrose, dextrin, ethyl cellulose, and the like) in water. The coating solution may be dispensed or applied by spraying, pouring, or ladeling the solution onto a mass of centers. To ensure uniformity, and to prevent the coated centers from adhering to each other during the drying process, coating solutions are normally applied while the centers are being intermixed in a coating vessel. Typically, a current of drying gas is introduced into the vessel simultaneously with or subsequent to application of the coating material. The most commonly used gas is air, which is usually heated.
Efforts have been made to identify the preferred drying gas temperature, flow rate, moisture content, vigorousness of center intermixing and dispersion of the coating liquid to improve the efficiency of coating processes and the consistency and quality of the coated products produced thereby. Conventional processing parameters for each of these process variables have been identified. For example, to obtain an appropriate rate of drying required for formation of a high quality coating, the drying gas (air) has a dew point of about 5xc2x0 C. (5.4 g water/kg dry air). The flow rate of the drying gas through the coating vessel is considered to be dependent upon the weight of the mass of the centers to be coated/dried and the type of coating vessel used. For example, for coating a mass of centers weighing about 400 kilograms (kg) using a rotatable drum, flow rates of about 1,800 ft3/min to about 4,000 ft3/min may be used, however, for a mass of centers weighing about 2,500 kg flow rates of about 8,000 ft3/min to about 10,000 ft3/min may be used. In contrast, coating a mass of centers weighing about 30 kilograms (kg) using a fluidized bed, flow rates of about 350 ft3/min to about 1,000 ft3/min may be used. The temperature or temperature range to which the drying gas may be adjusted is a predetermined temperature/temperature range that is dependent upon the thermal stability of the center to be coated. For example, for centers that are insensitive to high temperatures, that is, for centers that will not readily melt or degrade at high temperatures (e.g., greater than about 50xc2x0 C.), the drying gas temperature is typically maintained at a temperature of about 50xc2x0 C. to about 85xc2x0 C. In contrast, if the centers are thermally sensitive, that is, melt at a relatively low temperature (e.g., less than about 50xc2x0 C.) or are otherwise unstable at elevated temperatures or mildly elevated temperatures, the drying gas temperature is typically maintained at a temperature or within a range of temperatures that is below the melting or degradation temperature of the center.
There have been continuing attempts to refine coating processing operations. For example, Futter, U.S. Pat. No. 4,168,674 discloses a process whereby the bed temperature of the batch of tablets being coated in a rotating coating pan is sensed by a sensor. The coating process is monitored such that if during any one cycle there is a preselected difference between the sensed temperature and a reference temperature stored in memory devices, a signal will be produced. This process suffers from the disadvantage that the coated tablets may be damaged by repeated collisions with the sensor. Likewise, the sensor may be damaged by repeated collisions with the intermixing tablets in the rotating pan.
Yoakam, U.S. Pat. No. 4,554,887, discloses a coating apparatus with computer control in which a several operating parameters can be controlled via a computer. For example, the spray rate, air inlet temperature, air temperature in the coating pan, exhaust air temperature, coating pan speed, air flow, dew point, and composition of the spray material may be automatically controlled for producing coated tablets. Latini, U.S. Pat. No. 5,495,418, discloses a coating system which controls fewer processing variables to control the drying/spray cycles, such as the beginning and ending dry times and the length of spraying/amount of coating solution dispensed onto the centers.
None of the above methods or apparatus provide operators with the ability to monitor the actual progress of the coating process inside the coating apparatus and to rapidly adjust or modify the coating process based on such monitoring information. Accordingly, it would be advantageous to provide a simple and efficient coating method, apparatus and system for monitoring of the progress of the coating process by monitoring characteristics of the coated tablets or centers inside of the coating apparatus and a method by which the coating process could be readily controlled. It would be particularly advantageous to provide such a coating method, apparatus and system with which such a coating process could be conducted in a rapid and cost effective manner.
This invention relates to an improved method, apparatus and system for producing a shell coating on a mass of centers, comprising a coating vessel, through which is provided a current drying gas, in which an intermixed mass of centers is coated with a coating solution. An apparatus of this invention comprises a temperature sensor for measuring the temperature of the surface of the coated centers of the mass. Another apparatus of this invention comprises a moisture sensor for measuring the moisture content of the surface of the coated centers of the mass. A method of this invention comprises drying coated centers by measuring the temperature of the coated centers of the mass using the temperature sensor and adjusting the temperature of the drying gas to maintain the surface temperature of the coated centers of the mass at a predetermined temperature. Another method of this invention comprises measuring the moisture content of the surface of the coated centers using the moisture sensor and drying the coated centers until the moisture content of the surface of the coated centers is about 0% to about 30% water, by weight.